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Experiments in extreme focal reduction

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#76 Gavster

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Posted 24 July 2018 - 10:56 PM

Nice. FOV seems to be just right.

I assume you use Ha filter(?)

Yes both photos were using a 6nm ha filter. I calculate the fov to be 2.6 degreees with this focally reduced setup.



#77 GeezerGazer

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Posted 25 July 2018 - 07:03 PM

Gavin, nice job on those!  Really nice tracking, and exposure in the Galaxy S9 is terrific. Two things come to mind.  With your images, the FoV does show outer edge star aberrations in about the outer 10%... but less than I saw in my images using my f:5 ST120 with a 55P and .5x or .8x reducers.  And Mike Lockwood's much faster Newt also showed outer field star distortions with the 55P (using no extra reducer).  But I am wondering if the 55P and .8x reducer would work better in my f:7 140 refractor...  Maybe trying to reduce an already fast or semi-fast scope is counter productive.  Further testing should help clarify. 

 

Second thing I noticed is that there is some vignetting in both of your images.  Do you think that is from the reducer?  Do you have any vignetting with the 55P when used without the reducer in the same scope?  I am searching for zero outer field distortions AND zero vignetting with widest true field... with my existing scopes.  

 

Last question Gavin... have you used a 32mm or shorter FL eyepiece in afocal with the reducer and do you see any outer field aberrations with that combination?  I ask because I noted less outer field aberrations with my 40mm + reducer than with the 55mm + reducer. Aberrations were still present, but the difference was severity.  


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#78 Gavster

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Posted 26 July 2018 - 04:37 AM

Gavin, nice job on those!  Really nice tracking, and exposure in the Galaxy S9 is terrific. Two things come to mind.  With your images, the FoV does show outer edge star aberrations in about the outer 10%... but less than I saw in my images using my f:5 ST120 with a 55P and .5x or .8x reducers.  And Mike Lockwood's much faster Newt also showed outer field star distortions with the 55P (using no extra reducer).  But I am wondering if the 55P and .8x reducer would work better in my f:7 140 refractor...  Maybe trying to reduce an already fast or semi-fast scope is counter productive.  Further testing should help clarify. 

 

Second thing I noticed is that there is some vignetting in both of your images.  Do you think that is from the reducer?  Do you have any vignetting with the 55P when used without the reducer in the same scope?  I am searching for zero outer field distortions AND zero vignetting with widest true field... with my existing scopes.  

 

Last question Gavin... have you used a 32mm or shorter FL eyepiece in afocal with the reducer and do you see any outer field aberrations with that combination?  I ask because I noted less outer field aberrations with my 40mm + reducer than with the 55mm + reducer. Aberrations were still present, but the difference was severity.  

Thanks Ray. 

Regarding the vignetting, the phone camera certainly exaggerates this imo. Visually the vignetting is not obvious to me. However, yes I think it’s the reducer doing this.

Attached is a photo of the Eagle taken on the same night using a 32mm plossl which doesn’t show vignetting or edge distortion imo.

And an older one from January with the Tec and the 55mm plossl and can’t see vignetting and edge distortion here.

 

However, I like using the reducer to get the extra speed for better visual views even with the slight edge distortion and vignetting. It does seem like the 55mm plossl and reducer combo will have this issue though.

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#79 Jeff Morgan

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Posted 26 July 2018 - 10:26 AM

Regarding the vignetting, the phone camera certainly exaggerates this imo. Visually the vignetting is not obvious to me. However, yes I think it’s the reducer doing this.

 

More great shots!

 

If you have a case on your phone (and in the era of $1000 phones, most of us do), the thickness of the case will cause vignetting. Definitely better to remove the case.

 

I've only dabbled a little with this, and have yet to experiment with the second (2x) lens on my iPhone X. It could be interesting.


Edited by Jeff Morgan, 26 July 2018 - 10:26 AM.


#80 Gavster

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Posted 26 July 2018 - 10:51 AM

More great shots!

 

If you have a case on your phone (and in the era of $1000 phones, most of us do), the thickness of the case will cause vignetting. Definitely better to remove the case.

 

I've only dabbled a little with this, and have yet to experiment with the second (2x) lens on my iPhone X. It could be interesting.

Thanks. I was pleased to get a decent shot of the pillars of creation in the eagle. Looking to get much better shots of the horsehead later this year.

Yes I remove the phone case before any photos to try to get the lens as close to the adapter opening as possible to avoid as much as possible vignetting. All my recent photos are using the f1.5 lens on the samsung s9.


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#81 GeezerGazer

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Posted 26 July 2018 - 12:52 PM

Gavin,

Thank you for the additional photos they are very helpful.  They represent the best afocal images I've seen.  When I look at the image of the Eagle, taken afocally using your 32 Plossl, I see no distortion in the outer edge stars... until I zoom in.  It is very slight, but the edge-of-field stars do look slightly elongated compared to those closer to the center FoV.  To me, this is a huge improvement over my results with only the 55 Plossl in the f:5 ST 120 or in the images I've seen using very fast reflectors with a 55P.  The edge aberration is so slight, it is simply a non-issue when using the 32 Plossl.  Would it be correct to guess that this was a single, 10 second exposure? 

 

The conundrum is your older image of the Horsehead/Flame.  Using your f:7 apo with the 55P, shows ZERO edge distortion; even zoomed in.  In all of the afocal images I've seen to date using a 55P, in a faster (faster than f:7) focal ratio scope, edge distortion was present, including my f:5 120.  But here, in your f:7, there is none.  Do you have the exposure time on this image?  It was taken before your Galaxy S9 arrived; so with your iPhone.  Metadata on the original should reveal the exposure time.  This has me wondering if the edge distortion we see with the elongated stars at the edge of the field has to do with field rotation using tracking alt/az mounts COMBINED with some degree of field curvature and/or astigmatism.  If true, differences in exposure time could conceivably account for changes in the severity of outer field aberrations that are visible; length of star trails in that outer edge.  

 

When the smoke clears (smoke from forest fires in the Sierra Mtns. closed Yosemite yesterday), I'll test using different exposure times when tracking with my AZ Pro.  I might be all wrong about this, and it could be nothing.  But at this point, it is worth investigating.

 

As previously noted, the issue of precise focus when field curvature is present, may also be playing a part in the severity of the outer edge aberrations seen in photos.  So this needs a test with images to see differences; focusing on-axis; and then focusing on the edge-of-field stars... and maybe somewhere in between.  

 

Regarding removal of camera cases for Phonetography, I solved the issue of vignetting by drilling the open aperture in my phone bracket, to increase the diameter by 3/64" or about 1mm.  I also chamfered/relieved the bottom edge of the open aperture which also helped to mitigate vignetting.  The phone camera lens simply needs a large enough aperture in the bracket to see the phosphor screen without vignetting caused by an intervening protective camera case pushing the camera lens farther from the bracket.  If I drop my iPhone in the dark, I want my Pelican case on it.  Before I relieved the hole in my phone bracket, the phone camera and bracket required perfect alignment to take a photo without vignetting.  I took several photos that showed a darker side, occurring when the phone was slightly bumped off center.  Relieving the clear aperture hole provides a little leeway to overcome that issue and still leave the phone case on the camera.  


Edited by GeezerGazer, 26 July 2018 - 01:02 PM.

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#82 Gavster

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Posted 26 July 2018 - 03:52 PM

Ray,

Yes the second older photo was done with nightcap on my iPhone so was probably 0.5 secs exposure time stacked for 15 seconds. Maybe there is something with the longer 10 sec exposures of the Samsung and the alt az tracking - certainly I had issues with star shapes with my az gti Mount which is not as accurate as the panther TTS mount that I was using for these photos.

I think my observing sessions are over for a bit until the next new moon but I will try again on various objects then (and maybe even from my dark 21.3 sqm site with one of my bigger fracs ;))


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#83 GlennLeDrew

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Posted 26 July 2018 - 08:30 PM

Note that vignetting can happen when the camera lens is moved *too close* to the eyepiece. The optimal placement is where the iris/nodal point of the camera lens is in plane with the exit pupil. Phone cameras and their lenses are generally pretty compact, shallow constructs. It is awfully easy to push their lenses well inside the exit pupil. Doing so clips at least some of the more steeply converging cylindrical bundles from the outer field.


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#84 GeezerGazer

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Posted 26 July 2018 - 10:05 PM

Gavin, I don't actually think the Galaxy S9 ten second exposure has anything to do with the outer field aberrations; I see the same kind of outer field aberration, but more severe, from my ST 120, using my iPhone with stacked 1/4 second images.  I do think we are narrowing in on understanding the limits of afocal and afocal with a reducer... but we'll see what additional testing reveals.  It is clear from your results and from Moshen's prime + reducer images, that improvements are possible without significant aberrations.  The scope focal ratio and the focal reduction value used for a wider/brighter image both seem to play a significant role in how much aberration actually shows up visually or in an image.  Longer exposures or stacks of shorter exposures probably does not play a part in the outer field star trails; but I'll do some simple tests to verify.  

 

Thanks Glenn, I have a ScopeTronix 40mm eyepiece with T2 connection at the top of a sliding sleeve and will use it to experiment with distance settings between the eyepiece and the NVD.  Phone camera distance to the NVD ocular is more problematic.  I'll see what I can do for a systematic adjustment.  I read the Wikipedia explanation of an optical system nodal point 4 times and still do not fully understand what the nodal point is.  

 

I have not reached a positive conclusion yet, but it appears that the experiments in extreme afocal reduction are limited in large part, by the characteristics of the eyepiece used.  TeleVue eyepieces are tested for use in systems down to f:4... an f:2.6 optical system, is probably a little much for a Plossl.  It is not surprising to me that it may be producing field curvature; it may be more surprising that it performs as good as it does.

 

Gavin, it would be interesting if you can:

1.  Replicate (on a different target) the results of your Horsehead/flame image with zero outer field distortion... to determine what hardware and settings are necessary to accomplish this feat.  

2.  Take three images of the same rich star field on your tracking mount with the 55 Plossl and .75x reducer, each with progressively longer exposure time (compensating by adjusting ISO), say 1, 5 & 10 second images just to see if the outer edge distortions are effected by the length of exposure on an alt/az tracking mount.  I'll try to do the same.

 

I will be out of country pretty much until late Aug, so won't have access to my equipment for testing until then.  But this is now on my list.  I'd really like to see where best reduction performance meets minimal or zero aberrations.  Gavin, you might already have found it!  

 

We'll have to wait for Jeff or Eddgie to take a few images using their astrographs anyway to see what full field illumination looks like in a phone photo.  I look at jdbastro's images and see what I think IS full field illumination.  

 

I think most of my remaining tests should be conducted with both the ST 120 (2" focuser) and the TEC 140 (3.5" focuser) to see how much impact the larger focuser has on vignetting... using the same diagonal/reducer/adapters & night vision on both scopes.  Moshen suggested that some vignetting may be attributable to the diameter and length of the focuser draw tube.  I know this is a possibility in the ST 120.  Comparison photos will be forthcoming... in time.  


Edited by GeezerGazer, 26 July 2018 - 10:41 PM.

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#85 Jeff Morgan

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Posted 26 July 2018 - 10:34 PM

Note that vignetting can happen when the camera lens is moved *too close* to the eyepiece. The optimal placement is where the iris/nodal point of the camera lens is in plane with the exit pupil. Phone cameras and their lenses are generally pretty compact, shallow constructs. It is awfully easy to push their lenses well inside the exit pupil. Doing so clips at least some of the more steeply converging cylindrical bundles from the outer field.

 

<sigh>

 

It just doesn't get easy does it?

 

wink.gif



#86 GeezerGazer

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Posted 26 July 2018 - 11:40 PM

<sigh>

 

It just doesn't get easy does it?

 

wink.gif

 

It will get easy if we figure it out!    Everything we use has parameters for optimal connections to something else.   For afocal, there is an extra piece in the construct.  But for instance finding the optimal phone to NVD ocular distance should be fairly straight forward.  In this thread, we have considered problems including vignetting, outer field distortions, and full field illumination... each with more than one cause.  I think when we are finished we will have a pretty good idea where to look within our own, different optical systems to remedy some of the issues.  To many, the visual distortions created by taking an f:5 scope down to an f:2 are acceptable for the change in perspective/scale.  For some of us, the distortions are a little more than we want to accept.  I like a clean image from my scope and NVD; testing so far has revealed my tolerance level to field distortions and vignetting.  

 

Basically, there are different levels of performance that are acceptable to each of us.  Moshen has found a way of reducing his prime image at .65x, providing a really beautiful image.  And Gavin is getting remarkable results using afocal without a reducer; and acceptable results with a reducer IN FRONT OF a long (55mm) afocal lens.  The earlier tests through the ST 120 showed what worked in my scope to satisfy my needs.  And others looking at those results, Gavin's and Moshen's images should take note as to what combination of tools were used to achieve the results and which results are acceptable and achievable to them.  I'm sure that jdbastro, eddgie, cnoct and others here have all done this type of experimental testing to see what is acceptable.  The difference here is that we are using images to convey the results.  There are more questions to be answered, but answers are coming, even if slowly.  And the posts being made are still on topic.  We are still in investigative mode!  smiley-char145.gif

 

Jeff, aren't you the ultimate experimenter?  ... Mr. Wiley Coyote!  Dobsonian.gif



#87 Jeff Morgan

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Posted 27 July 2018 - 12:28 AM

It will get easy if we figure it out!    Everything we use has parameters for optimal connections to something else.   For afocal, there is an extra piece in the construct.  But for instance finding the optimal phone to NVD ocular distance should be fairly straight forward.  In this thread, we have considered problems including vignetting, outer field distortions, and full field illumination... each with more than one cause.  I think when we are finished we will have a pretty good idea where to look within our own, different optical systems to remedy some of the issues.  To many, the visual distortions created by taking an f:5 scope down to an f:2 are acceptable for the change in perspective/scale.  For some of us, the distortions are a little more than we want to accept.  I like a clean image from my scope and NVD; testing so far has revealed my tolerance level to field distortions and vignetting.  

 

Basically, there are different levels of performance that are acceptable to each of us.  Moshen has found a way of reducing his prime image at .65x, providing a really beautiful image.  And Gavin is getting remarkable results using afocal without a reducer; and acceptable results with a reducer IN FRONT OF a long (55mm) afocal lens.  The earlier tests through the ST 120 showed what worked in my scope to satisfy my needs.  And others looking at those results, Gavin's and Moshen's images should take note as to what combination of tools were used to achieve the results and which results are acceptable and achievable to them.  I'm sure that jdbastro, eddgie, cnoct and others here have all done this type of experimental testing to see what is acceptable.  The difference here is that we are using images to convey the results.  There are more questions to be answered, but answers are coming, even if slowly.  And the posts being made are still on topic.  We are still in investigative mode!  smiley-char145.gif

 

Jeff, aren't you the ultimate experimenter?  ... Mr. Wiley Coyote!  Dobsonian.gif

 

lol.gif

 

Mostly that Wile E thing is for having things blow up in my face.

 

But OK, I'm in the game!


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#88 Gavster

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Posted 27 July 2018 - 02:15 AM

Gavin, I don't actually think the Galaxy S9 ten second exposure has anything to do with the outer field aberrations; I see the same kind of outer field aberration, but more severe, from my ST 120, using my iPhone with stacked 1/4 second images.  I do think we are narrowing in on understanding the limits of afocal and afocal with a reducer... but we'll see what additional testing reveals.  It is clear from your results and from Moshen's prime + reducer images, that improvements are possible without significant aberrations.  The scope focal ratio and the focal reduction value used for a wider/brighter image both seem to play a significant role in how much aberration actually shows up visually or in an image.  Longer exposures or stacks of shorter exposures probably does not play a part in the outer field star trails; but I'll do some simple tests to verify.  

 

Thanks Glenn, I have a ScopeTronix 40mm eyepiece with T2 connection at the top of a sliding sleeve and will use it to experiment with distance settings between the eyepiece and the NVD.  Phone camera distance to the NVD ocular is more problematic.  I'll see what I can do for a systematic adjustment.  I read the Wikipedia explanation of an optical system nodal point 4 times and still do not fully understand what the nodal point is.  

 

I have not reached a positive conclusion yet, but it appears that the experiments in extreme afocal reduction are limited in large part, by the characteristics of the eyepiece used.  TeleVue eyepieces are tested for use in systems down to f:4... an f:2.6 optical system, is probably a little much for a Plossl.  It is not surprising to me that it may be producing field curvature; it may be more surprising that it performs as good as it does.

 

Gavin, it would be interesting if you can:

1.  Replicate (on a different target) the results of your Horsehead/flame image with zero outer field distortion... to determine what hardware and settings are necessary to accomplish this feat.  

2.  Take three images of the same rich star field on your tracking mount with the 55 Plossl and .75x reducer, each with progressively longer exposure time (compensating by adjusting ISO), say 1, 5 & 10 second images just to see if the outer edge distortions are effected by the length of exposure on an alt/az tracking mount.  I'll try to do the same.

 

I will be out of country pretty much until late Aug, so won't have access to my equipment for testing until then.  But this is now on my list.  I'd really like to see where best reduction performance meets minimal or zero aberrations.  Gavin, you might already have found it!  

 

We'll have to wait for Jeff or Eddgie to take a few images using their astrographs anyway to see what full field illumination looks like in a phone photo.  I look at jdbastro's images and see what I think IS full field illumination.  

 

I think most of my remaining tests should be conducted with both the ST 120 (2" focuser) and the TEC 140 (3.5" focuser) to see how much impact the larger focuser has on vignetting... using the same diagonal/reducer/adapters & night vision on both scopes.  Moshen suggested that some vignetting may be attributable to the diameter and length of the focuser draw tube.  I know this is a possibility in the ST 120.  Comparison photos will be forthcoming... in time.  

Ray, just to be clear - the focus here is on the phone images produced rather than the actual views at the eyepiece? I think small  distortion/vignetting that shows up on images is very often not apparent at all when looking through the NV eyepiece.



#89 GlennLeDrew

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Posted 27 July 2018 - 12:23 PM

Geezer,

Here's what I think is a good way to understand the concept of the nodal point of a lens...

 

Imagine you're wanting to shoot video of model scenery, such as for an HO scale railroad. You want to include shots from a fixed perspective while panning the camera, and you desire for the result that would obtain by a suitably miniaturized to scale camera/tripod.

 

In a typical mounting most of us use, the camera lens protrudes fairly well forward of the pan head's azimuth axis. And so when panning, not only does the optical axis sweep through some angle, the lens also physically sweeps some distance in an arc. The visual result is of the POV floating laterally, as though you are simulating a shot from a flying drone looking sideways. Far removed from the desired result of simulating a tiny camera on a tiny tripod operated by a tiny photographer. wink.gif

 

For instance, suppose that while you sweep through an angle of, say, 60 degrees, your lens nodal point also physically sweeps through an arc of length 4 inches. At the scale of HO scenery that's equivalent to, what, 25 feet or so (I forget the scale figure)? The result screams a hack's attempt at miniature photography, and the lateral sweep in perspective really highlights the smallness of the subject.

 

You get around this by sliding your camera rearward, until the lens's iris (nodal point) lies exactly above the pan head's azimuth axis. Now when you pan, there is zero lateral displacement of the POV. You accurately simulate the extremely small camera in your miniaturized world.

 

The nodal point is where any and all chief rays from any and all angular offsets from the optical axis cross the optical axis. This coincides with the plane of the iris (if present.)


Edited by GlennLeDrew, 27 July 2018 - 12:27 PM.

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#90 GeezerGazer

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Posted 27 July 2018 - 09:53 PM

Ray, just to be clear - the focus here is on the phone images produced rather than the actual views at the eyepiece? I think small  distortion/vignetting that shows up on images is very often not apparent at all when looking through the NV eyepiece.

I think we should look at both Gavin.  If there is a substantial difference between the visual impact and imaging impact, we should note it.  We know that vignetting is less obtrusive visually, but that it shows readily in images.  I think notes attached to images to describe the difference is sufficient.  Honestly, I did not look for full field illumination issues before I saw the results that Ed pointed out in my images.  But it is there to see, and visually, the difference is equally apparent... if we know what to look for.  And if we find a way to minimize the impact, all the better.  It's like with H-a filters that produce band shift... if you keep the H-a target in the center FoV, you never see band shift... unless you look at a really big nebular target that extends beyond the FoV.  Then, band shift becomes a problem, visually and in photos.  



#91 GeezerGazer

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Posted 27 July 2018 - 09:57 PM

The nodal point is where any and all chief rays from any and all angular offsets from the optical axis cross the optical axis. This coincides with the plane of the iris (if present.)

Glenn, thank you for that explanation.  With the miniature example, everything falls into place.  Thanks for keeping track of us here!  bow.gif



#92 Jeff Morgan

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Posted 07 August 2018 - 10:32 AM

What I'd REALLY like to see here is a couple of photos through Ed's Boren Simon or Jeff's Tak Epsilon to SEE the difference an astrograph makes in field illumination. 

 

Ok, finally got all the components together last night and tried a little Phonetography while I waited for help from Serge at AstroDevices. BTW, Serge is a great guy! Called me within minutes of my email even though the issue was not his Nexus 2/DSC kit, rather SkySafari needing to be reset to factory settings.

 

Anyway here is the North American Nebula, three identical Nite Cap exposures except for the times. Photo 1:

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#93 Jeff Morgan

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Posted 07 August 2018 - 10:33 AM

Photo 2:

 

 

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#94 Jeff Morgan

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Posted 07 August 2018 - 10:34 AM

Photo 3 (btw, these were all shot 12nm H-alpha as I recall):

 

 

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#95 Jeff Morgan

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Posted 07 August 2018 - 10:37 AM

Looking at the left edge of the frame where the nebula does not seem to drop off in brightness. Visually, brightness at edges is excellent when I slew across the object.

 

Looks like I caught a satellite in the first exposure.


Edited by Jeff Morgan, 07 August 2018 - 10:38 AM.


#96 Starman81

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Posted 07 August 2018 - 12:10 PM

Jeff, those look great! Can you give some information on scope/focal speed/magnification and observing site (SQM/darkness)? 



#97 Jeff Morgan

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Posted 07 August 2018 - 10:06 PM

Jeff, those look great! Can you give some information on scope/focal speed/magnification and observing site (SQM/darkness)? 

I don't have a SQM device, but the rest is easy:

 

Takahashi Epsilon e180, 180mm f/2.8. With the Mod 3, 18.5x. True field approximately 2.1 degrees. 

 

The observing site is my back yard, about 6 miles north of downtown Prescott. Bortle 5 sky trending rapidly to Bortle 6. Yellow zone. 

 

Don't know how obvious it is from the file size limitations of CN, but when I flip rapidly between them on my computer the noise reduction is very easy to see from 1 to 2 to 3. I had the gain on my Mod 3 up to where it was easy to see the nebula on the iPhone screen. Probably too high! Also need to spend some time with NiteCap. Just kind of guessing on the settings.

 

Not the best composition, my mind was on getting all the new equipment (G11, DSC's, Nexus 2, and rotating tube rings) all set properly. The good news is all the equipment is "tuned" and ready for the next clear night!

 

Edit: Just noticed something in the Info boxes - the file size of the TIFF gets smaller with longer exposures.


Edited by Jeff Morgan, 07 August 2018 - 10:07 PM.


#98 GeezerGazer

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Posted 07 August 2018 - 11:18 PM

Jeff, really nice photos.  Did you use prime or afocal... and if afocal, what eyepiece?  If these are afocal, I'm not seeing any of the outer field distortion that has been present in most afocal NV images.  

 

Your longest (15.7s) stacked exposure gave NiteCap enough time to even out the exposure very nicely, eliminating most of the noise visible in the first (short exposure) image.  I like the added star detail that the 12nm filter provides.  If this was one of your first attempts with your iPhone, my hat is off to you!  waytogo.gif


Edited by GeezerGazer, 07 August 2018 - 11:35 PM.


#99 Jeff Morgan

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Posted 07 August 2018 - 11:52 PM

Jeff, really nice photos.  Was this with your Tak Epsilon, on your new Losmandy?   And, did you use prime or afocal... and if afocal, what eyepiece?  If these are afocal, I'm not seeing any of the outer field distortion that has been present in most afocal NV images.  

 

Your longest (15.7s) stacked exposure gave NiteCap enough time to even out the exposure very nicely, eliminating most of the noise visible in the first (short exposure) image.  I like the added star detail that the 12nm filter provides.  If this was one of your first attempts with your iPhone, my hat is off to you!  waytogo.gif

 

Why thank you. But as they say ... even the blind squirrel finds an occasional acorn.

 

They were prime focus shots, Epsilon on the Losmandy G11. The connection is made from a Tak "Wide T" Canon mount on the Epsilon and a Canon -> C adapter on the Mod 3. Easy Peasy. 

 

The G11 was really purchased for a Ceravolo HD145 I was able to acquire back in May. At 50x per inch the Ceravolo is whispering in my ear "Is that all you got?". It needed an upgrade from it's GM-8.

 

I've been using the Epsilon on a HalfHitch mount, which is quite nice for the NV world of low to mid powers. But the equatorial opens up some Phonetography possibilities. It kind of begs the question: how long can NiteCap go on its stacking runs? Where is the point where it no longer matters?


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#100 GeezerGazer

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Posted 08 August 2018 - 12:56 AM

NiteCap will stack up to 60 minutes, but with astro phonetography, precise tracking is more likely the limiting factor.  You will need to run a few tests on a single target to get a feel for the time needed for stacking.  But for my needs, stacking is pretty short, 30s or less; mine are usually around 15s.  But I'd suggest you take a series of photos, increasing exposure times incrementally so you can see where further improvement is not obtained with additional stacking.  I'm not a pixel peeper and my images are just for my notes.  

 

Below is a comparison, using an image I took on 6-4-18, with my 60mm achro, f:3.8, for it's big FoV, using a 5nm H-a filter, ISO 3712, 1/2s, stacked 30s on my iPhone 6+.  You can see how the 5nm filter attenuates the star field in my image; and how the edges of the FoV show vignetting and/or non-full-field illumination.  I used your third (longest) exposure for the comparison.  Even though I was using a high ISO at 3712 and a very narrow H-a filter, 30 seconds of stacking helped a lot to rid the image of much of the noise a phone camera sensor will show.  It is a remarkable technology.  

 

Gavster and RVA_Chris both use Samsung Galaxie 8 & 9 phones, which allow for a single exposure up to 10 seconds.  I think, based on their results, the single long exposure (not stacked) yields a superior image.  The short (1/3s to 1/2s maximum) exposures allowed on the iPhone are good when stacked in NiteCap, but it would be great if iPhone allowed a longer single exposure.  

 

N.A. nebula.jpg




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